1. Field of the Invention
The present invention relates to a reversible thermosensitive coloring material, which achieves a relatively colored state or a relatively discolored state utilizing a coloring reaction of an electron donating coloring compound with an electron accepting compound. In addition, the present invention also relates to a reversible thermosensitive recording material, which includes a reversible thermosensitive coloring material and which can reversibly record and erase an image by controlling the heat energy applied to the reversible thermosensitive coloring material.
2. Discussion of the Background
Thermosensitive recording materials, which record images utilizing a coloring reaction of an electron donating coloring compound (hereinafter referred to as a coloring agent or a leuco dye) with an electron accepting compound (hereinafter referred to as a color developer), are well known. Such thermosensitive recording materials are broadly used for output media of facsimiles, word processors and various measuring instruments as well as various tickets and price labels. In addition, recently the thermosensitive recording materials are used for magnetic thermosensitive recording media such as prepaid cards and reward cards.
Since these thermosensitive recording materials utilize an irreversible coloring reaction, the recording materials cannot reversibly record and erase images. Therefore the thermosensitive recording materials cannot be reused, resulting in increase of wastes. In the case of cards, new information is recorded in a space of a recording area of the cards. Therefore, when the recording area of a card is filled with recorded information (i.e., when the recording area has no recording space), the card has to be replaced with a new card. Accordingly, a need exists for a reversible thermosensitive recording material, which can reversibly record and erase images many times in order to save materials.
In attempting to fulfill the need, various reversible thermosensitive recording media have been proposed, and some of the recording media are marketed now.
For example, published unexamined Japanese patent applications Nos. (hereinafter referred to as JP-As) 63-107584 and 04-078573 have disclosed polymer-type reversible thermosensitive recording media utilizing a physical change, in which a transparent state and an opaque state are reversibly achieved.
In addition, dye-type reversible thermosensitive recording media utilizing a chemical change have been proposed. For example, JP-A 60-193691 discloses a dye-type reversible thermosensitive recording medium using a combination of gallic acid with phloroglucinol. JP-A 61-237684 discloses a dye-type reversible thermosensitive recording medium using compound (e.g., phenolphthalein and thymolphthalein) as a color developer. JP-As 62-138556, 62-138568 and 62-140881 have disclosed dye-type reversible thermosensitive recording media having a recording layer including a homogeneous mixture of a coloring agent, a color developer and a carboxylic acid ester. JP-A 63-173684 discloses a dye-type reversible thermosensitive recording medium including an ascorbic acid derivative as a color developer. JP-As 02-188293 and 02-188294 disclose a salt of bis(hydroxyphenyl)acetic acid or gallic acid with a higher aliphatic amine as a color developer.
JP-A 05-124360 proposes a reversible thermosensitive recording medium using a coloring agent and a color developer. It is described therein that by using a combination of a specific color developer (e.g., an organic phosphoric acid compound, an aliphatic carboxylic acid, or a phenolic compound, which has a long chain aliphatic hydrocarbon group) with a coloring agent (e.g., a leuco dye), a color image can be easily formed and erased reversibly when properly controlling the heating and cooling conditions. The reversible thermosensitive recording medium can reversibly achieve a colored state and a non-colored state many times, and the colored state and non-colored state can be stably maintained at room temperature. With respect to the reversible thermosensitive recording medium, JP-A 06-210954 further discloses that a phenolic compound having a long chain aliphatic hydrocarbon group having a specific structure is used as a color developer.
The above-mentioned reversible thermosensitive recording media tend to have the following drawbacks:    (1) the erasing speed is so slow that it takes a long time to perform rewriting;    (2) recorded images are not satisfactorily erased; and    (3) recorded images have poor heat stability.
In attempting to remedy the drawback, i.e., to improve the practicality of the reversible thermosensitive recording media, and the usability of the reversible thermosensitive recording media for small-sized low energy printers, there is a need for a reversible thermosensitive recording material which can perform erasure of a former image and record of a new image at the same time using a thermal printhead. In attempting to fulfill the need, various coloring/discoloring control agents or discoloring promoters have been developed. For example, JP-As 09-048175, 09-290563 and 11-070731 have disclosed use of a long chain alkyl compound to attempt to improve the erasability of the recording media or both of the erasability and stability of recorded images.
Further, JP-A 05-294063 discloses a reversible thermosensitive recording medium including a discoloring promoter, such as fatty acids, salts of fatty acids, waxes, higher alcohols, esters of phosphoric acid, benzoic acid, phthalic acid and oxy acids, silicone oils, liquid crystal compounds, surfactants, etc.
These reversible thermosensitive recording materials are not satisfactory in view of high speed erasability (i.e., a residual image is present even after image erasure) when a thermal printhead is used as a recording/erasing device.
In addition, it is proposed to use ionic salts as discoloring promoters. For example, JP-A 08-108627 discloses a combination of a specific coloring developer with a quaternary ammonium salt or an alkylene oxide. However, these recording materials have unsatisfactory high-temperature preservability.
JP-A09-300820 uses onium salts as discoloring promoters. JP-A 10-044607 uses quaternary ammonium salts having a complex aromatic ring as discoloring promoters. JP-A 2000-263946 discloses combinations of a discoloring agent having a polar group (e.g., onium salts, and quaternary ammonium salts having a complex aromatic ring) with a specific color developer. JP-As 2000-313171 and 2001-047749 have disclosed combinations of a quaternary ammonium salts with a specific color developer.
However, these reversible thermosensitive recording media including such an ionic discoloring promoter have a drawback in that recorded images are erased at a relatively low temperature, and therefore recorded images have poor preservability when preserved at a relatively high temperature of about 50° C. In addition, the reversible thermosensitive recording materials are not satisfactory in view of high-speed erasability (i.e., a residual image is present even after image erasure).
Further, JP-As 07-285270 and 07-285271 disclose compounds having both an acidic group (such as carboxyl groups and sulfo groups) and a basic group (such as amino groups) as discoloring promoters. However, these reversible thermosensitive recording media are also not satisfactory in view of high-speed erasability and high-temperature preservability of recorded images.
Because of these reasons, a need exists for a reversible thermosensitive recording material which can record a high-contrast image while having a good combination of high-speed erasability and high-temperature preservability.